In the more familiar technique for producing semi-finished nickel-based alloy components, particularly those used for critical applications, such as rotating disks for jet engines, the raw materials are melted in a vacuum induction furnace of 20,000 to 35,000 pounds or more. The raw materials can include virgin metal and scrap to achieve the nominal alloy composition and to reduce the overall cost of the process. A large ingot is formed from the melting process. This ingot usually contains defects of at least three types; voids, macrosegregation and inclusions. Subsequent processing is generally used to eliminate or minimize the defects caused by the previous processing. For example, electroslag refining is commonly used to remove the oxide and sulfide and slag inclusions. This process is described in detail on pages 82-84 of a text on metal refining entitled "Superalloys, Super composites and Superceramics" edited by John K. Tien and Thomas Caulfield and published by Academic Press. The product of electroslag refining has significantly lower concentrations of oxides and sulfides than the product from the vacuum induction melting process. It is also largely free of voids and slag inclusions.
A problem arises in electroslag refining of large diameter ingots because of the formation of a relatively deep melt pool during processing. The deep pool results in excessive macrosegregation and microsegregation, manifest as "freckles", and in less desirable microstructures.
One way to overcome this deep pool problem is to reduce the diameter of the ingot being processed, but this adversely affects the economics of the process and limits the maximum product size. Another way is to incorporate a subsequent melting step in combination with the electroslag refining. The subsequent melting step is vacuum arc remelting, and is a well known process, known to produce a relatively shallow melt pool and to produce a better microstructure. Thus, for any given alloy, it is generally agreed that a larger diameter ingot of acceptable quality can be produced by vacuum arc remelting as compared to electroslag remelting. Nonetheless, there are size limitations for vacuum arc remelting which, if exceeded, produce ingots with unacceptable levels of macrosegregation and microsegregation. Ingots from vacuum arc remelting may also contain inherent defects known as "white spots".
Cast ingot is processed via conventional mechanical working techniques to yield wrought stock with improved microstructures and properties. Such a combination of mechanical working may involve a combination of steps of forging, rolling and drawing to lead to a relatively smaller grain size. The thermomechanical processing of large ingots requires a large space on the factory floor and requires large and expensive equipment as well as large and costly energy input. In addition, the yield of final product may be low due to losses at each of the many steps involved.
For some alloys, metal producers manufacture powder prior to the metal working procedures in order to obtain the required microstructure and properties. In a common powder processing route, gas atomization is employed to produce metal powder which is subsequently screened. A selected portion of the screened powder is then encapsulated in a steel can and the can is hot isostatically pressed or extruded to consolidate the powder into a useful form. The consolidated billet may be processed by other conventional working steps to bring the consolidated product into final wrought form. Such processing of powder material is conventional and has been described in several publications.
An alternative to the previously described processing routes is to spray form the product in a process described in an number of U.S. patents, including U.S. Pat. Nos. 3,909,9231; 4,926,923; 4,779,802; 5,004,153; 5,310,165 as well as a number of other patents. The spray forming process is typically used to produce semi-finished product in the form of round billets, tubes or rings. In the spray forming process, a stream of molten metal or metal alloy is atomized with inert gas and the resulting spray is directed at a collector where the atomized droplets re-coalesce to form a high density product. The collector is rotated and simultaneously oscillated and may be moved away from the spray to maintain a constant spray distance. Rapid solidification of the droplets occurs during flight and on deposition thereby resulting in a fine, uniform microstructure without macrosegregation.
The potential advantages of spray forming processes have been described in the literature. In summary, spray forming produces a fine scale microstructure characteristic of rapid solidification in a single processing step from molten metal to product. T. Andersen, et al., in a paper given at the 1st European Conference on Continuous Casting in Florence, Italy, 1991, describes the commercial Osprey.TM. process. Leatham et al., in U.S. Pat. No. 4,938,275, describe certain important parameters in the Osprey process and discuss the importance of extracting heat from the atomized particles. Leatham describes a procedure whereby heat is extracted from the atomized particles by supplying gas to the atomizing device under carefully controlled conditions and by controlling the further extraction of heat after deposition.
Generally, the spray forming process has been gaining acceptance in industrial usage because of the excellent macrostructural and microstructural quality, and particularly because it involves fewer processing steps and has a cost advantage over conventional powder metallurgical techniques.
To achieve the best product quality, it has long been recognized that efforts must be taken to: 1) minimize porosity within the deposited metal; and 2) assure conditions of rapid solidification on the collector surface. Optimization of the spray process involves many factors, one of which is the temperature of the collector surface. If the collector surface is too cold, large amounts of undesirable porosity will result. On the other hand, if the surface is too hot, undesirable coarse or segregated structures may result.
One method of maintaining the temperature of the collector is to spray the next layer onto the collector as soon as the previous layer has cooled sufficiently to achieve structural integrity with the collector. Keutgen et al., U.S. Pat. No. 5,054,539, discloses an improvement in the process enabling the production of round bars of axial symmetry by spraying the molten metal onto a collector at such a rate that the collector is completely covered after a single 360.degree. rotation of the collector. This requires cycle times, a cycle being the time between successive passes of the spray head over the same area of the collector, sufficiently short to prevent over-cooling of the previously deposited layer. A convenient method of accomplishing this objective is to rotate the collector at a sufficiently rapid speed to prevent overcooling.
Another technique of maintaining the temperature of the collector in the proper temperature range is to add or subtract heat from the system. Leatham et al. discuss the importance of extracting heat from the atomized particles. Ikawa, in U.S. Pat. No. 5,305,816, discusses the importance of adding heat to the system by adhering a molten metal to the collector prior to spraying the atomized metal onto the collector.
Other techniques have been proposed to increase or maintain adherence of the sprayed metal to the collector or mandrel. In U.S. Pat. No. 5,143,139, Leatham describes a method of spray forming which assures a strong bond between the surface of the collector and the spray droplets. Leatham proposes two techniques for assuring strong bonding between the sprayed metal and the collector surface. First, the collector surface is grit blasted before spray deposition. Leatham also proposes preheating the collector using a plasma heating means disposed immediately upstream of the deposition surface.
Cheskis et al. in U.S. Pat. No. 5,343,926, discloses using two nozzles to achieve a low porosity between the collector and the metal. The first nozzle directs an initial deposit onto the collector with a sufficient amount of molten metal to fill the inherent interstices between the splatted droplets while the mostly solid metal stream from the second nozzle has sufficient solids content to ensure that the shape is maintained.
One of the continuing limitations of the spray forming process is that the diameter of the spray formed preform is generally limited by physical constraints of the system. Utilizing conventional processing techniques, cylindrical preforms are limited to diameters of 12 inches or less and rings or tubular preforms are limited to about 36 inches maximum outer diameter. An increase in preform diameter will improve the economics of the process, make it even more competitive with consolidated powder and conventional processing, and open new markets for larger products.
One method of obtaining larger diameter ingots is disclosed by Forrest et al. in U.S. Pat. No. 5,472,038. Forrest discloses the use of multiple sprays for large diameter bars (e.g., 12 to 24 inches in diameter).
Other methods have been proposed to resolve the preform size problem. All of these methods are cumbersome and complex. The maximum preform sizes achieved using current technologies are about 12 inches in diameter for a single nozzle and about 20 inches in diameter for dual nozzle apparatus. There still exists a need to produce large size preforms easily and inexpensively.
The physical limitation which currently prevents spray forming of larger preforms is attributed to the need to deposit each spray formed particle onto the thin semi-liquid layer on the surface of the spray formed preform. Using present processing techniques, a spray formed preform is built up by directing the spray of molten metal onto the end or face of a rotating surface. As the spray deposit is built up, the preform is gradually withdrawn to maintain a constant distance from the spray nozzle to the surface of the preform. The preform can be oriented at any angle from the horizontal to vertical. To produce optimum quality spray formed product, particularly the highest density deposit, the operating conditions are set so that each particle will be deposited onto the semi-liquid layer which is maintained on the end surface of the preform.
To maintain this critical, semi-liquid layer as the preform diameter is increased, the rotational speed must increase so that complete solidification does not occur from the time a given segment exits the spray cone, is rotated approximately 360.degree. and comes under the molten metal spray again. However, the rotational speed can only be increased a finite amount before the billet becomes unstable and breaks away from its mountings due to centrifugal forces. Also, if the speed is too great, centrifugal force causes the semi-liquid material on the surface to be flung off. Contrarily, if the speed is too slow, the semi-liquid material will solidify before a given segment re-enters the spray. The end result in either case is that the spray is deposited onto a solid layer. If the surface of the preform is not sufficiently liquid, the resulting billet will contain undesirable porosity.